1. Field of the Invention
The present invention relates to an apparatus for detecting a relative displacement between a scale and a detecting head.
2. Description of the Background Art
A Japanese Patent Application First Publication No. Showa 54-30060 (published on Mar. 6, 1979) exemplifies a previously proposed relative displacement detecting apparatus.
That is to say, the previously proposed relative displacement detecting apparatus includes a pair of detecting heads, a scale, and detecting head signal processing circuits using a two channel processing method; namely, a first channel detecting head signal processing circuit including a first of the pair of detecting heads and a second channel detecting head signal processing circuit including a second of the pair of detecting heads.
The previously proposed relative displacement detecting apparatus described above further includes: an oscillator which generates and outputs a signal having a reference frequency f; an excitation circuit which generates and outputs an excitation signal .epsilon. of a sinusoidal wave or pulse wave having the reference frequency in response to the signal from the oscillator; a pair of preamplifiers which amplify the corresponding one of the detection signals of the pair of detecting heads, the detection signal being an equilibrium modulated signal; a phase shifter which shifts the equilibrium modulated signal derived from a first one of the pair of preamplifiers by 90.degree.; an adder which adds the equilibrium modulated signal derived from a second one of the pair of preamplifiers and the 90.degree.-phase shifted equilibrium modulated signal derived from the phase shifter and outputs a phase modulated signal of the sinusoidal wave therefrom; and a band pass filter which eliminates unnecessary signal components from the phase modulated signal from the adder.
As described above, since the phase modulated signal output from the adder usually includes higher harmonic wave distortions, the band pass filter of a higher order is provided so as to provide a desired phase modulated signal from the output signal of the adder.
However, the band pass filter causes a source of time delay in the relative displacement detecting apparatus and disturbs a size of the apparatus from miniaturization and a low cost thereof.